Fluoropolymers are used in broad industrial fields such as automobile industry, semiconductor industry, and chemical industry as a raw material of such as a sealing material used under severe conditions due to showing excellent chemical resistance, solvent resistance and heat resistance.
Preparation of these fluoropolymers is performed by emulsion polymerizing or suspension polymerizing fluoroolefins. Usually, a surfactant is used in emulsion polymerization, and when an amount of a surfactant in used is larger, the number of polymer particles generated in the initial period of emulsion polymerization increases, its polymerization speed increases, and production efficiency of a fluoropolymer is improved. When a surfactant is used in a large amount, however, there is a tendency that the surfactant lowers various physical properties such as water resistance and the like of the obtained fluoropolymer. Therefore, development of a process for preparing a fluoropolymer, wherein polymerization can be efficiently conducted in the presence of a small amount of a surfactant which does not cause an adverse effect on various physical properties of a fluoropolymer, has been expected so far.
Under such circumstances, a fluoropolymer is prepared by using a straight chain aliphatic sulfonate surfactant for the purpose of substitution of expensive ammonium perfluorooctanoate generally used in emulsion polymerization of a fluoropolymer (for example, see U.S. Pat. No. 6,512,063). This process, however, has a problem of having a small number of generated particles.